For any who were curious how stuff's going RE: the updates and fixes, I've started a dedicated update thread for the Forum Overhaul. I'll be posting progress, taking feedback and suggestions, and so on over yonder! If you've reported or report bugs in this thread, I'll be working 'em into the log too, so we good there.

Alignment tags on a single line don't result in a blank line, unlike most tags. They sort of eat the blank line, leaving one less line break.

"
[b]
b
[/b]
"

Becomes

"
b

"
(Notice the blank lines between the quotation marks and the "b", that's where the tags are.)

But

"
[align=left]
a
[/align]
"

Becomes

"

a

"
(The lines with the tags on them just kind of disappear, never to be seen again)

Mismatched tags act... strangely. Say you have the tags [b]1[u]2[/b]3[/u]. You'd expect it to print out something like this -- 1[u]23[/u] -- right? The 2 and 3 are bolded, because they're within the [b] tags. The [u] tags don't work -- nothing is underlined and the tags print as plaintext -- because the front tag can't access the end tag from within the [b] tags.

Or maybe formatting can actually overlap without being completely contained, and it would just end up like this -- 123 -- everything nice and simple and working fine.

Nope.

The end tags take on each other's roles, and you get this -- 123 -- that is, what you would have gotten if the tags were balanced in the beginning, with the bold on the outside.

It'd probably not be easier to migrate to a new system, no. That'd break at least as many features, and may change a lot of things we won't be able to fix to be like they used to be.

If it helps, I have a good idea what causes these formatting tag problems; when the forum backend turns your BBcode into HTML, there are a few different phases of text-parsing done that sometimes conflict, and which sometimes create unexpected effects. For one example, the raw text of a post is changed by the built-in MyBB parser, which sometimes adds line breaks, and even permanently changes the text of tags you type when a post is submitted, which can be seen when editing a post with such a tag; a [center] tag becomes [align=center] for instance.

MyBB doesn't make it terribly easy to recode their built-in text parser; it's hidden unless you have back-end access, and changes may be overwritten by updates. The stuff Admins are intended to use is strapped on top in an Admin-created Custom BBCode Tag system in the Admin control panel. These tags are easy to code, but less powerful (they just turn certain BBCode Tags text into HTML tags, but can't do more complicated scripting stuff). Any BBCode Tags we create using their built-in system also can't be nested the way built-in tags can be, among other weird idiosyncrasies. This is how most content plugins (video embedding, some alignment tags, etc.) work.

Forum Admins also have access to more complex-to-create but more powerful Javascript Plugins. These aren't subject to the same restrictions since their functionality is all made from scratch, but it's also considerably more work to code and implement 'em. For example, this is how Spoilers work, and why they can be nested.

Anyway, I still need to investigate the backend stuff to see what I can do to change it; unfortunately, it would probably need to be recoded, or at least manually copied over, any time we update the forum backend. But I think it's entirely fixable, once I understand the system better. Still gotta figure out the best way to do that though.

I can't reproduce the issue for some reason but while updating cloudsong I deselected the source button on the post preview window and it changed the BBC code to HTML code, then when I tried to unclick the source button it stopped previewing the images in the post preview. When attempting to hit the source button again it'd just add extra extraneous BBC code tags wrapped around the text effectively breaking the formatting I was doing.

Just sort of went around it by opening a new post tab.

I tried a bunch of ways to reproduce this issue like inserting images and bolding text, which was all I was doing with the original post but didn't get the same results. Just mentioning it here just in case its relevant to anyone who runs into it in the future.